The preparation of alkylene carbonates by reacting an alkylene oxide and carbon dioxide is well known. The general conditions for the reaction are the use of temperatures in the approximate range of 100.degree.-250.degree. C. Superatmospheric pressures of about 10-300 atmospheres are employed. A reaction temperature of about 160.degree.-200.degree. C. and a pressure of 50-150 atmospheres are usually preferred. The reactants are used in about equal molar proportions with the carbon dioxide normally in slight excess.
Known catalysts for the reaction include inorganic bases such as sodium hydroxide and sodium carbonate and organic nitrogen bases such as tertiary amines, quaternary ammonium bases, and salts of these nitrogen bases such as their carbonates and halides. For example, aliphatic tertiary amines such as trimethylamine, aromatic tertiary amines such as pyridine and quinoline, quaternary ammonium hydroxides such as tetraethyl ammonium hydroxide, trimethyl benzyl ammonium hydroxide, dialkyl piperidinium hydroxide, and the carbonates, bicarbonates, and halides of such hydroxides are all known to catalyze the reaction. Catalyst concentrations of 0.1-5 percent based on the weight of alkylene oxide are conventional.
Other catalysts disclosed in the patent literature are anion exchange resins containing quaternary ammonium chloride groups (U.S. Pat. No. 2,773,070), hydrazine or the hydrohalide salt thereof (U.S. Pat. No. 3,535,341) and quanidine and its salts (U.S. Pat. No. 3,535,342). Catalysts known to the art generally are effective for the purpose and they provide fairly high conversions of the reactants and generally good yields of the desired cyclic carbonates. These yields usually are about 70-90 percent of the theoretical. The latter two patents claim conversions and yields each in excess of 95%.
The manufacture of alkylene carbonates is effectively accomplished in conjunction with an alkylene oxide plant. Thus, for example, ethylene carbonate production is advantageously located in the proximity of an ethylene oxide plant.
The ethylene oxide obtained from the direct oxidation of ethylene is usually not pure enough to be employed as a feed to an ethylene carbonate plant without prior purification. It has now been discovered that when ethylene oxide is recovered from the effluent of such a plant by absorbing it in ethylene carbonate, the ethylene oxide together with the carbon dioxide absorbed can be employed without further purification as feed to a process for making ethylene carbonate. Thus, a simple stripping and drying of these gases from the ethylene carbonate absorbent provides reactants of acceptable purity for this process.